Photochromism is the reversible function of a certain compound that it changes its color swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and returns to its original color when it is put in the dark by stopping its exposure to light. A compound having this property is called “photochromic compound” and used as a material for photochromic plastic lenses.
For the photochromic compound used for this purpose, the following properties are required: (I) the degree of coloration at a visible light range before ultraviolet light is applied (initial coloration) should be low, (II) the degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high, (III) the speed from the time when the application of ultraviolet light is started to the time when the color optical density reaches saturation (to be referred to as “color development sensitivity” hereinafter) should be high; (IV) the speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high, (V) the repeat durability of this reversible function should be high, and (VI) the solubility in a monomer composition which will become a host material after curing of the photochromic compound should be high so that its dispersibility in the host material in use becomes high.
As the photochromic compound which can satisfy these requirements, there are known chromene compounds having an indeno(2,1-f)naphtho(1,2-b)pyran structure as the basic skeleton (refer to a pamphlet of International Laid-Open WO99/15518 and a pamphlet of International Laid-Open WO2001/60811).
It is preferred that a photochromic plastic lens comprising the photochromic compound should develop a color of a neutral tint such as gray or brown. A color of a neutral tint is obtained by mixing together several different kinds of photochromic compounds which develop different colors. More specifically, it is obtained by mixing a yellow to red photochromic compound (yellow compound) having a maximum absorption at 430 to 530 nm with a purple to blue photochromic compound (blue compound) having a maximum absorption at 550 to 650 nm at the time of color development.
However, when color control is carried out by this method, various problems occur due to the difference in photochromic properties between the compounds which have been mixed together. For example, when the repeat durability of the yellow compound is lower than that of the blue compound and the photochromic plastic lens is used for a long time, the developed color gradually changes to a color with a strong blue tint.
When the color development sensitivity and fading speed of the yellow compound are lower than those of the blue compound, there occurs a problem that the color during color development has a strong blue tint and the color during fading has a strong yellow tint.
It is considered that this problem can be solved by using a compound which has two or more absorption maximums at the time of exposure and develops a color of a neutral tint by itself (double peak compound). It is known that a yellow compound is generally inferior to a blue compound in durability. Therefore, a compound having a higher yellow color optical density (having a maximum absorption wavelength at 430 to 530 nm) than the blue color optical density (having a maximum absorption wavelength at 550 to 650 nm) is desired as the double peak compound (the ratio of the yellow color optical density to the blue color optical density in the double peak compound may be referred to as “double peak characteristic” hereinafter).
As the photochromic compound having two or more absorption maximums at the time of color development (double peak compound), there are known compounds represented by the following formulas (A) to (D).
However, these compounds have room for the improvement of the following points. That is, a chromene compound represented by the following formula (A) (refer to a pamphlet of International Laid-Open WO01/19813) has a low fading speed and low repeat durability though its double peak characteristic is high.

A chromene compound represented by the following formula (B) (refer to a pamphlet of International Laid-Open WO03/044022) has smaller absorption at 430 to 530 nm than absorption at 550 to 650 nm and low double peak characteristic.

Although a chromene compound represented by the following formula (C) (refer to a pamphlet of International Laid-Open WO05/028465) has excellent double peak characteristic and practical levels of color optical density and fading speed, the end of its absorption spectrum (to be referred to as “absorption end” hereinafter) goes beyond 420 nm into the visible range. Therefore, it has room for improvement because its initial coloration is slightly large.

Further, US2009/0309076A1 discloses a chromene compound represented by the following formula (D). However, the compound has room for improvement as its absorption at 430 to 530 nm is smaller than absorption at 550 to 650 nm and its double peak characteristic is low.
